Recently, environment and energy problems are more and more significant in the world, and the demands to save energy in various systems are more and more increasing. The motor drive system is required to have multiple functions, high performance and energy-saving and the operation with variable speed is developing. The variable magnetic flux motor, in which magnetization of the permanent magnet can change directly, has been thought out as a solution of the technology (see Patent Document 1).
Two kinds of magnets, i.e., a variable flux magnet and a fixed flux magnet, are used in the variable magnetic flux motor. The fixed flux magnet is required to have properties, i.e., a high residual magnetic flux density and a high coercivity which will not deteriorate due to inverter current on the condition of high torque, and the requirement is the same as that of a conventional permanent magnet synchronous motor. In addition, the variable magnetic flux motor is demagnetized through a current magnetic field in order to inhibit the increase of back electromotive force that will prevent roll controlling during revolution at a high speed, and the motor is remagnetized through the current magnetic field when the torque is needed during the operation. Thus, the variable flux magnet has the function of demagnetization and magnetization. When the magnetization state is controlled by a small applied magnetic field, it is expected that much less consumption of the variable magnetic flux motor can be realized. Further, in order to obtain a high output and a high efficiency of the variable magnetic flux motor, the variable flux magnet is required to provide a magnetic flux equal to that of the fixed flux magnet. That is, the variable flux magnet is required to have magnetic properties, i.e., a low coercivity and a high residual magnetic flux density.
For example, a Sm—Co based permanent magnet has been disclosed as a variable flux magnet (Patent Document 2). The improvement of the efficiency of the motor can be realized by combining the structure of an Nd—Fe—B based permanent magnet with a high coercivity, as a fixed flux magnet, in the way of not being demagnetized by load current.